A galvanic cell type oxygen sensor including a positive electrode, a negative electrode, an electrolyte solution, and an oxygen permeable membrane is small and light, and is further operable at normal temperature and inexpensive. Thus, the sensor is used in various fields for the check of the state of oxygen deficiency in ship's holds and manholes, the detection of the concentration of oxygen in a medical instrument such as an anesthesia machine or an artificial respirator, and other purposes.
FIG. 6 illustrates an ordinary sectional structure of a galvanic cell type oxygen sensor that has been hitherto put into various practical uses. In FIG. 6, reference number 1 represents a first holder lid (inner lid); 2, an O-ring; 3, a protective membrane for preventing the adhesion of rubbish and dust onto a first oxygen permeable membrane, and the adhesion of a water membrane thereto; 4A, the first oxygen permeable membrane; 4B, a catalyst electrode; 5, a positive current collector; 6, a positive leading wire; 7, an electrolyte solution; 8, a negative electrode; 9, a holder body; 10, a second holder lid (outer lid); 11, a bore through which the electrolyte solution is to be supplied; 12, a bore into which the leading wire is inserted; 13, a positive current collector holding region; 14, a correcting resistance; and 15, a temperature compensating thermistor. The catalyst electrode 4B and the positive current collector 5 constitute a positive electrode 45. The first holder lid 1 and the second holder lid 10 constitute a holder lid 101.
The principle of the operation of the galvanic cell type oxygen sensor is as follows: Oxygen that has passed through the first oxygen permeable membrane 4A, through which oxygen is selectively caused to permeate, the amount of the permeation of oxygen being restricted in accordance with the reaction of the cell, is reduced in the catalyst electrode 4B, which is capable of reducing oxygen electrochemically, so that electrochemical reactions described below are caused between the catalyst electrode 4B and the negative electrode 8 by aid of the electrolyte solution 7.
When the electrolyte solution is acidic, the following are caused:
Positive electrode reaction: O2+4H++4e−→2H2O
Negative electrode reaction: 2Pb+2H2O→2PbO+4H++4e−
Total reaction: 2Pb+O2→2PbO
When the electrolyte solution is alkaline, the following are caused:
Positive electrode reaction: O2+2H2O+4e−→4OH−
Negative electrode reaction: 2Pb+4OH−→2 PbO+2H2O+4e−
Total reaction: 2Pb+O2→2PbO
The case of the acidic electrolyte solution and that of the alkaline electrolyte solution are different from each other in electric charge carrier. However, in either one of these cases, a current corresponding to the concentration of oxygen is generated between the catalyst electrode 4B and the negative electrode 8. The current generated by the positive electrode reaction on the catalyst electrode 4B is collected into the positive current collector 5, which is brought into compressive contact with the catalyst electrode 4B, and then led through the positive leading wire 6 to the outside. The current flows through the correcting resistance 14 and the temperature compensating thermistor 15 into the negative electrode to be converted into a voltage signal. As a result, a voltage is gained as an oxygen sensor output. Thereafter, the resultant output voltage is converted to the concentration of oxygen in a well-known manner, and then the oxygen concentration is detected.
When such a conventional galvanic cell type oxygen sensor is allowed to stand still in the atmosphere or any other oxygen-containing atmosphere, the above-mentioned electrochemical reaction is naturally generated so that the electrode material is unfavorably consumed. Thus, there remains a problem that when the storage period of the oxygen sensor is long before the use thereof, the use lifespan of the oxygen sensor is largely shortened.
Thereagainst, JP-A-07-190984 discloses a technique of wrapping an oxygen gas sensor with an oxygen gas barrier transparent film while sealing up a deoxidizer and an oxygen-detecting agent therein.